1. Field of the Invention
Generally, the present invention relates to the field of metrology and metrology tools used in the fabrication of integrated circuits, and, more particularly, to optical measurement tools and methods for determining characteristics of circuit elements during the various manufacturing stages.
2. Description of the Related Art
The manufacturing of integrated circuits requires the formation and interconnection of a huge number of individual circuit elements, such as transistors, capacitors, resistors and the like, on a small chip area. In producing the circuit elements, a plurality of material layers are successively deposited on a substrate and they are patterned in accordance with design requirements by sophisticated photolithographic and etch techniques. As the dimensions of the individual circuit elements decrease and the complexity of the circuit increases with every new generation of integrated circuits, tolerances for the individual process steps involved in fabricating the circuits have to be maintained within very strictly set ranges. In order to monitor process quality during the various manufacturing stages, great efforts are made to provide measurement results representing the effect and the quantity of the individual process steps in the most efficient manner possible. Consequently, a plurality of measurement tools, also referred to as metrology tools, are provided as part of, or separate from, the process line to allow the adjustment or readjustment of process parameters to form the circuit elements meeting the specification set by the design rules. Among the measurement methods used to determine the characteristics of circuit features, those techniques which allow the gathering of highly precise measurement results in a non-destructive manner are gaining in importance. For example, in many situations, the exact determination of a layer thickness is essential and a plurality of metrology tools have been developed for this task. Among others, so-called spectroscopic ellipsometers or photometers are preferably used to provide a light beam of specified optical characteristics and to detect a secondary light beam reflected by a substrate bearing the material layer, the thickness of which is to be determined, to obtain the required information. Recently, such optical metrology tools have also been used to determine properties of circuit features patterned in a material layer. To this end, a periodic structure of test features is formed at a specified location on the substrate and is exposed to a light beam of known optical characteristics.
In this context, it should be noted that the terms xe2x80x9copticalxe2x80x9d and xe2x80x9clight beamxe2x80x9d refer to any type of radiation, e.g., microwaves, infrared light, visible light, x-rays and even charged particles, having an appropriate wavelength so as to carry information on the periodic structure upon being scattered therefrom.
A detector is positioned to receive the light beam scattered by the periodic structure to obtain measurement spectra, from which information related to the periodic structure may be extracted. Many types of apparatus may be used for providing an appropriate light beam and for detecting the diffracted beam. For example, U.S. Pat. No. 5,867,276 describes a so-called 2-xcex8-Scatterometer, wherein the angle of incidence of a light beam is continuously varied by synchronously rotating the sample and the detector. Additionally, this document describes a scatterometer system utilizing a rotating block to translate a light beam emitted from a light source to different points of the entrance aperture of a lens to illuminate the substrate at different angles of incidence. Moreover, this document describes a scatterometer with a fixed angle of incidence utilizing a multi-wavelength illumination source to create and obtain the required information from the diffracted multi-wavelength beam. From the information contained in the measurement spectrum, the optical and dimensional properties of individual elements that form the periodic structure and the thickness of underlying films may be extracted, for example, by statistical techniques. The parameters of interest of the periodic structure may include the width of lines, if the periodic structure contains lines and spaces, the sidewall angles, and other structural details.
In principle, information indicative of values of these parameters may be extracted by computing an intensity distribution of the scattered beam with respect to wavelength, location in space, polarization, and the like from the basic design of the periodic structure, the optical characteristics of the materials of which the periodic structure is formed, and from the basic physical equations (Maxwell""s equations) describing the interaction of the radiation with matter. The results, obtained by computation, may then be compared to actual measurement data and the difference between the two sets of data is indicative of a variation of one or more parameters. For instance, a deviation of the sidewall angle of a line within a grid pattern may lead to a subtle intensity variation compared to the computed spectrum, and the difference in intensity may then be assigned to a specific value of the sidewall angle. The computation of a corresponding set of reference spectra, however, requires a fairly large amount of computational power and computation time and thus, commonly, computing the reference data is carried out in advance and reference spectra or data for a given type of periodic structure are stored in a so-called library.
In addition to the scatterometers described above, metrology tools that allow an optical measurement of layer thickness, such as spectroscopic ellipsometers and photometers, are used more frequently for scatterometry due to their broad availability. In order to reliably obtain precise film thickness measurement results, the properties of these metrology tools have to be continuously monitored and maintained within very strict margins, since a very subtle variation, for instance, of the light source and/or the detector, may result in an intolerable degradation of measurement performance. Thus, automatic measurement cycles are commonly carried out on a regular basis with internal film thickness standards to monitor and possibly readjust the metrology tool. Thus, when used for scatterometry, any re-calibration and/or readjustment and/or drift of the hardware of the tool may also affect measurement results of the scatterometry measurement, although, in principle, the scatterometry results may be considered as xe2x80x9cabsolute,xe2x80x9d since they are obtained on the basis of the fundamental physical equations. The effect of any hardware variation of the metrology tool on scatterometry results is therefore conventionally monitored by periodically measuring a set of reference wafers, which are also referred to as xe2x80x9cgolden wafers.xe2x80x9d The verification of the current hardware adjustment of the tool, therefore, periodically requires a user""s attention and time to substantially avoid any hardware drift that may jeopardize the reliability of scatterometric measurement.
In view of the above situation, there exists a need for reliably monitoring the status of metrology tools used for scatterometry in a time-efficient and effective manner.
Generally, the present invention is directed to apparatus and methods used in scatterometry, wherein a pitch calibration standard, i.e., a simple periodic standard pattern and a corresponding reference data library, is integrated into the metrology tool so that any variations in the hardware of the tool may be detected and monitored in a time-efficient manner.
According to one illustrative embodiment of the present invention, a scatterometer system comprises a light source configured to emit a light beam of predefined optical characteristics and a detector configured to receive a light beam scattered by a sample. Moreover, a substrate holder is provided that is adapted to receive the sample and hold it in place during a measurement cycle. Additionally, the scatterometer system comprises a pitch calibration station including a pitch calibration standard and a library data unit adapted to provide reference data indicative of the pitch calibration standard.
According to still a further illustrative embodiment of the present invention, a method of calibrating a scatterometer comprises providing a pitch calibration standard and establishing a reference data library for the pitch calibration standard. Moreover, measurement data of the pitch calibration standard is obtained and is compared with the reference data library.
In yet a further illustrative embodiment of the present invention, a method of operating a scatterometer comprises starting a self-test routine of the scatterometer. The self-test routine comprises obtaining measurement data from a pitch calibration standard and comparing the measurement data with reference data of the pitch calibration standard. The scatterometer is released for further measurement when a result of the comparison is within a predefined allowable range.